We have recently demonstrated that the catecholamine (CA) synthesizine enzyme tyrosine hydroxylase (TH) is expressed in a large and previously undescribed population of adult neurons located in layer I of the cerebral cortex. These neurons have not yet been observed in the developing cortex. However, when the cortex of E13 rats is dissociated and placed into culture, we find several thousand TH immunoreactive neurons. Expression of the enzyme is restricted to neurons which have completed cell replication, occurs only in cells at a critical stage in their development (E13-E16) and gradually declines in culture as some neurons acquire GABAergic traits. Our findings in culture, combined with the anatomical localization of TH cells in the adult cortex, suggest, but do not prove, that TH is being expressed in a group of neurons known as Cajal-Retzuis (C-R) cells. To date, the fate and function of C-R cells remains a mystery. The fact that many of their processes project to the pial membrane raises the possibility that blood vessels, acting as targets, may in some way regulate the phenotypic expression of transmitter traits in these cells. Indeed, in our preliminary experiments, we found that the porportion of TH immunoreactive cortical neurons was dramatically increased (20 fold) by growing the cells with a soluble factor(s) derived from muscle cells of vascular origin as well as from other muscle sources. The present study seeks to determine 1) Whether TH cells are indeed C-R neurons; 2) Whether C-R cells die or transdifferentiate into cells of another neurotransmitter phenotype; 3) Whether other CA traits are expressed by these cells in vivo and in vitro; 4) Whether, during the critical period, TH expression is modified as a result of changes at the transcription or translation level; 5) The mechanism and nature of the epigenetic signal regulating the expression of TH in cortex; and 6) Whether these target-derived factors found in muscle also play a role in signally CA differentiation in vivo. These issues will be analyzed using a multidisciplinary approach which includes biochemical, autoradiographic, immunocytochemical and molecular biological techniques. In so doing, we expect to identify and characterize a new population of neurons expressing the CA enzyme TH and to further our understanding of the basic genetic and epigenetic principles which govern the choice of transmitter phenotype in brain neurons. Our hope is that these studies may provide some clues as to the processes operating during normal development of the CNS as well as in certain pathological conditions.